Viewfinder optical system

ABSTRACT

A viewfinder optical system includes an objective optical system for forming an image of an object, and an eye-piece optical system for converting rays of light from the image into substantially parallel rays of light, wherein the objective optical system is provided with an optical element having a diffraction optical surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to viewfinders for use in lens-shutter(leaf-shutter) cameras, single-lens reflex cameras or the like.

2. Description of Related Art

In recent years, there have been proposed a great number of lens-shuttercameras in which the size of a photographic optical system thereof hasbeen reduced to a compact form. Further, according to the reduction ofthe size of the photographic optical system, there is a growing demandfor an external viewfinder mounted on the camera, being small in sizeand having an excellent optical performance.

Generally, to achieve such a compact viewfinder, it is required that thenumber of constituent lenses thereof is reduced and, at the same time,the refractive power of each lens unit is strengthened. However, thisapproach causes production of various aberrations. As a result, anoptical system of high performance becomes difficult to attain.

Further, the recent trend is that ever-increasing numbers of the opticalparts of the external viewfinder for lens-shutter cameras aremanufactured by using plastic materials, in consideration of loweringthe cost and of improving the productivity. In such a situation, itbecomes important, despite no freedom of choosing glass materials, toattain improvements of the performance and of the compact form of theoptical system without sacrificing the cost and productivity.

In consideration of the optical performance of the viewfinder, whatdetermines whether the perception is good or bad when looking throughthe viewfinder depends largely on whether or not there are left smallresidual chromatic aberrations. If the amount of residual chromaticaberrations is large, color confusion appears at the contours of anobject being observed, and flare components are produced, giving no goodcomfortableness to the observer.

Particularly, for a wide-angle viewfinder to be realized, the lateralchromatic aberration increases with an increase of the field angle. Inthe situation that, as described above, emphasis is laid on the cost andproductivity, because the availability of optical materials is limitedand the number of constituent lenses is restricted to a smaller value, acorrection for the lateral chromatic aberration is difficult to carryout.

In general, on the premise that ordinary refracting optical elements aremade use of, improvements of the optical performance of the real-imageviewfinders are carried out by pairing up lens components of positiveand negative refractive powers. In order to further improve thecorrection of all aberrations in good balance, it is unavoidable toincrease the number of lens elements in the viewfinder.

Further, in order to reduce the number of lens elements while keepinggood optical performance, another effective means is to introduce anaspheric surface. However, as far as the chromatic aberrations areconcerned, because the correction for the chromatic aberrations dependson the difference in dispersion index between the materials of thepaired lens elements of positive and negative powers, no valuablecorrecting effect is expected from the use of the aspheric surface.

On account of such a situation, there have been made previous proposalsthat, without relying on the dispersion characteristics of the opticalmaterials, the amount of generation of chromatic aberrations can becontrolled by making use of a diffraction optical element in the opticalsystem, as disclosed in, for example, Japanese Laid-Open PatentApplications No. Hei 6-324262 and No. Hei 9-127322.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to provide a viewfinder optical systemhaving a diffraction optical element of a novel structural arrangement.

To attain the above object, in accordance with an aspect of theinvention, there is provided a viewfinder optical system, whichcomprises an objective optical system for forming an image of an object,and an eye-piece optical system for converting rays of light from theimage into substantially parallel rays of light, wherein the objectiveoptical system is provided with an optical element having a diffractionoptical surface.

In accordance with another aspect of the invention, there is provided aviewfinder optical system, which comprises an objective optical systemfor forming an image of an object, and an eye-piece optical system forconverting rays of light from the image into substantially parallel raysof light, wherein at least one of the objective optical system and theeye-piece optical system has a diffraction optical surface of rotationalsymmetry with respect to an optical axis, wherein, letting a distancefrom the optical axis be denoted by H, a reference wavelength be denotedby λ, and a phase coefficient with a term in the 2·i-th degree of H bedenoted by C2·i, a phase φ(H) of the diffraction optical surface isexpressed by the following expression:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.6 + . . . +C2·i·H.sup.2·i)

and wherein, letting a phase coefficient with a term in the seconddegree of H for the j-th diffraction optical surface be denoted by C2j,and a paraxial refractive power and Abbe number of the k-th opticalelement included in the viewfinder optical system be denoted by ψk andυk, respectively, the following condition is satisfied: ##EQU1## where nis the number of diffraction optical surfaces in the viewfinder opticalsystem, and m is the number of optical elements in the viewfinderoptical system.

In accordance with a further aspect of the invention, there is provideda viewfinder optical system, which comprises an objective optical systemfor forming an image of an object, and an eye-piece optical system forconverting rays of light from the image into substantially parallel raysof light, wherein a diffraction optical element is disposed on anoptical element which satisfies the following condition:

    |HD|>|H|

where H is a distance from an optical axis of a maximum zone in a bundleof on-axial light rays passing through optical elements which constitutethe viewfinder optical system, and HD is a distance from the opticalaxis of an off-axial principal ray which halves a bundle of off-axiallight rays having an effective maximum field angle.

These and further objects and aspects of the invention will becomeapparent from the following detailed description of preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic diagram showing the construction of a viewfinderoptical system according to each of embodiments 1 to 8 of the invention.

FIG. 2 is a schematic diagram of a secondary image forming type ofreal-image viewfinder.

FIG. 3 is a longitudinal section view of the construction of theembodiment 1.

FIG. 4 is a longitudinal section view of the construction of theembodiment 2.

FIG. 5 is a longitudinal section view of the construction of theembodiment 3.

FIG. 6 is a longitudinal section view of the construction of theembodiment 4.

FIG. 7 is a longitudinal section view of the construction of theembodiment 5.

FIG. 8 is a longitudinal section view of the construction of theembodiment 6.

FIG. 9 is a longitudinal section view of the construction of theembodiment 7.

FIG. 10 is a longitudinal section view of the construction of theembodiment 8.

FIG. 11 is a sectional view, in enlarged scale, of a diffraction opticalelement.

FIG. 12 is a graph showing diffraction efficiency relative towavelengths.

FIG. 13 is a sectional view, in enlarged scale, of a diffraction opticalelement having a laminated structure.

FIG. 14 is a graph showing diffraction efficiency relative towavelengths.

FIG. 15 is a sectional view, in enlarged scale, of another diffractionoptical element having a laminated structure.

FIGS. 16A to 16D are graphic representations of the aberrations in theembodiment 1.

FIGS. 17A to 17D are graphic representations of the aberrations in theembodiment 2.

FIGS. 18A to 18D are graphic representations of the aberrations in theembodiment 3.

FIGS. 19A to 19D are graphic representations of the aberrations in theembodiment 4.

FIGS. 20A to 20D are graphic representations of the aberrations in theembodiment 5.

FIGS. 21A to 21D are graphic representations of the aberrations in theembodiment 6.

FIGS. 22A to 22D are graphic representations of the aberrations in theembodiment 7.

FIGS. 23A to 23D are graphic representations of the aberrations in theembodiment 8.

FIG. 24 is a schematic diagram of a viewfinder optical system which hasno diffraction optical surface.

FIGS. 25A to 25D are graphic representations of the aberrations in theviewfinder optical system shown in FIG. 24.

FIG. 26 is a schematic diagram showing the construction of a viewfinderoptical system according to each of embodiments 9 to 11 of theinvention.

FIG. 27 is a schematic diagram of the arrangement of prisms for imageinversion with a thin lens system.

FIG. 28 is a schematic diagram showing a secondary image forming type ofreal-image viewfinder.

FIG. 29 is a longitudinal section view of the embodiment 9.

FIG. 30 is a longitudinal section view of the embodiment 10.

FIG. 31 is a longitudinal section view of the embodiment 11.

FIGS. 32A to 32D are graphic representations of the aberrations in theembodiment 9 in the wide-angle end.

FIGS. 33A to 33D are graphic representations of the aberrations in theembodiment 9 in a middle focal length position.

FIGS. 34A to 34D are graphic representations of the aberrations in theembodiment 9 in the telephoto end.

FIGS. 35A to 35D are graphic representations of the aberrations in theembodiment 10 in the wide-angle end.

FIGS. 36A to 36D are graphic representations of the aberrations in theembodiment 10 in a middle focal length position.

FIGS. 37A to 37D are graphic representations of the aberrations in theembodiment 10 in the telephoto end.

FIGS. 38A to 38D are graphic representations of the aberrations in theembodiment 11 in the wide-angle end.

FIGS. 39A to 39D are graphic representations of the aberrations in theembodiment 11 in a middle focal length position.

FIGS. 40A to 40D are graphic representations of the aberrations in theembodiment 11 in the telephoto end.

FIGS. 41A to 41D are graphic representations of the aberrations in aconventional example in the wide-angle end.

FIGS. 42A to 42D are graphic representations of the aberrations in theconventional example in a middle focal length position.

FIGS. 43A to 43D are graphic representations of the aberrations in theconventional example in the telephoto end.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the drawings.

FIG. 1 in schematic diagram shows the optical system of a real-imageviewfinder according to an embodiment 1 of the invention. In the opticalsystem, there are arranged an objective lens O1, an image re-forminglens O2, an inverting prism O3, a Pechan roof prism O4 and an eye-piecelens O5 in this order from the object side. In the embodiment 1, theobjective lens O1, the image re-forming lens O2 and the inverting prismO3 constitute an objective optical system, the Pechan roof prism O4constitutes an image inverting optical system, and the eye-piece lens O5constitutes an eye-piece optical system.

Incidentally, in the real-image viewfinder, the image inverting opticalsystem may be in the form of a Porro-prism or like prism, or mirrors.Otherwise, without using such prisms or mirrors, the real-imageviewfinder may be formed to a secondary image forming type of real-imageviewfinder, as shown schematically in FIG. 2.

FIG. 3 in lens block diagram shows the main constituent parts of theembodiment 1 of the real-image viewfinder with the image invertingoptical system of FIG. 1 in optically developed form. In FIG. 3, Sstands for a fixed stop, and E stands for the eye point. A surface R4 onthe object side of the image re-forming lens O2 is a diffraction opticalsurface.

FIG. 4 in lens block diagram shows the main constituent parts of theembodiment 2, in which a surface R5 on the image side of the imagere-forming lens O2 is a diffraction optical surface. FIG. 5 in lensblock diagram shows the main constituent parts of the embodiment 3, inwhich a surface R2 on the image side of the objective lens O1 and asurface R4 on the object side of the image re-forming lens O2 arediffraction optical surfaces.

FIG. 6 in lens block diagram shows the main constituent parts of theembodiment 4, in which a diffraction optical plate O6 is disposed inbetween the image re-forming lens O2 and the inverting prism O3. Asurface R7 on the image side of the diffraction optical plate O6 is adiffraction optical surface. In the case of the embodiment 4, thediffraction optical plate O6 is included in the objective opticalsystem. This applies also to the embodiments 5 to 8, which will bedescribed later.

Likewise, FIG. 7 shows the main constituent parts of the embodiment 5,in which a surface R6 on the object side of the diffraction opticalplate O6 is a diffraction optical surface. Also, FIG. 8 shows the main Aconstituent parts of the embodiment 6, in which both surfaces R6 and R7of the diffraction optical plate O6 are diffraction optical surfaces.

FIG. 9 in lens block diagram shows the main constituent parts of theembodiment 7, in which a surface R7 on the image side of the invertingprism O3 is a diffraction optical surface.

FIG. 10 in lens block diagram shows the main constituent parts of theembodiment 8, in which a diffraction optical plate O7 is disposed inbetween the objective lens O1 and the image re-forming lens O2. Asurface R4 on the object side of the diffraction optical plate O7 is adiffraction optical surface.

Such diffraction optical elements having a diffraction optical surfaceare manufactured by making up the grooves in a stepwise form called"binary optics" directly on the surfaces of the optical elements bylithography. Besides this method, there is another one, that that methodis used to take casts, thus producing replica gratings. Still anothermethod is to take a mold from which gratings are reproduced. Also, ifthe grooves are made up in the saw-tooth form, that is, kinoform, thediffraction efficiency increases. So, a diffraction effect close to theideal value can be expected.

Since the grooves of the diffraction optical surface are very fine inwidth and are spaced very close, the diffraction optical surface is verysusceptible to scratches and dust or foreign particles. For this reason,it is preferred that a surface other than the frontmost or rearmost oneof the viewfinder optical system be selected as a diffraction opticalsurface as far as possible.

In general, spherical lenses or like ordinary refracting opticalelements are used in making up the real-image viewfinder. Within thisframework, the optical performance is usually improved by combining twocomponents of opposite refractive powers. To correct all aberrations ingood balance, however, the total number of lens elements is caused toincrease largely.

To reduce the number of lens elements while keeping the improved opticalperformance, it is advantageous to introduce an aspherical lens.Concerning the chromatic aberrations, however, even with the help of theaspherical lens, no improved results can be expected, because thechromatic correction depends on the difference in dispersion indexbetween the materials of the paired-up components of negative andpositive refractive powers.

A diffraction optical surface of rotational symmetry with respect to theoptical axis can be expressed in terms of the distance H from theoptical axis by an equation for the phase φ(H) at the distance H, asfollows:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.6 + . . . +C2·i·H.sup.2·i)               (1)

where λ is the reference wavelength (spectral d-line), and C2·i is thephase coefficient with the term in the 2-i-th degree of H.

It is then made possible to control the refractive power of the paraxialzone and the chromatic aberrations for the reference wavelength inaccordance with variation of the phase coefficients C2 in the equation(1). The coefficient C4 and other coefficients of higher degree termsare usable for producing a similar effect to that of the asphericsurface in that the refractive power varies with the height of incidenceof light on the diffraction optical surface. At the same time, it ispossible to control the chromatic aberrations for the referencewavelength in accordance with variation of the height of incidence oflight. Moreover, in either case, for a small variation of the refractivepower, a large variation of the chromatic aberrations can be obtained.

The use of the diffraction optical element in the real-image viewfinderleads to providing of the possibility of using the aspheric surface incorrecting various aberrations while still permitting the chromaticaberrations to be controlled, which could not be corrected only byintroducing the aspheric surface.

By utilizing the above-described feature of the diffraction opticalelement, it is, therefore, possible to improve the optical performanceof the viewfinder with the limitation of the number of lenses to aminimum. That is, among all the aberrations that determine whether ornot the finder image is comfortably observed by the naked eye, thechromatic ones in particular are corrected well. A real-image viewfinderhaving an improved performance is thus achieved.

Another feature of the invention is that the viewfinder has an opticalsystem of widened field angle with an increased image magnification.Since, in this case, the objective optical system produces largelongitudinal and lateral chromatic aberrations, it is preferred that, asa means for correcting these aberrations, a location for the diffractionoptical element is taken within the objective optical system.

A furthermore feature of the invention is that, letting the coefficientin the second degree of the j-th diffraction optical surface be denotedby C2j, and the paraxial refractive power and Abbe number of the k-thoptical element included in the viewfinder optical system be denoted byψk and υk, respectively, the following condition is satisfied:

    0>Σ.sub.k=1.sup.m (ψk/υk)·Σ.sub.j=1.sup.n (2·C2j/3.45)                                     (2)

k-1 J-1 where n is the number of diffraction optical surfaces in theviewfinder optical system, and m is the number of optical elements inthe viewfinder optical system.

The first term (Σ_(k=1) ^(m) (ψk/υk) ) in the factor of the condition(2) is a simple formula representing the achromatic state of therefracting optical system in the viewfinder.

In general, the Abbe number (dispersion index) υd of the refractingoptical system is expressed by υd=(Nd-1)/(NF-NC), where Nd, NC and NFare the refractive indices for the d, C and F lines, respectively. Onthe other hand, the Abbe number υd of the diffraction optical element isexpressed by υd=λd/(λF-λC), where λd, λC and λF are the wavelengths ofthe d, C and F lines, respectively, so that there is obtained υd=-3.45.

The refractive power for the principal wavelength (d line) of thediffraction optical element in the paraxial first-order diffracted lightis expressed by ψ=-2·C2, where C2 is the coefficient of the term ofsecond degree in the equation (1) for the phase of the diffractionoptical element. Further, the refractive power ψ' for the arbitrarywavelength is expressed by ψ'=(λ/λo)·(-2·C2), where λ is the arbitrarywavelength and λo is the reference wavelength (principal wavelength).

The second term (Σ_(j=1) ^(n) (2·C2j/3.45)) in the inequality (2) simplyrepresents the chromatic correction in the diffraction optical element.So, the inequality of condition (2) is for canceling out the residualchromatic aberrations of the refracting optical system by thediffraction optical system.

To more effectively carry out the correction of chromatic aberrations,it is preferable to satisfy the following condition:

    0<|Σ.sub.j=1.sup.n (2·C2j/3.45)|-|ψ.sub.k=1.sup.m (ψk/υk)|                             (3).

When the inequality of condition (3) is violated, as the residualchromatic aberrations of the refracting optical system are corrected bythe diffraction optical system, either under-correction orover-correction results. So, this is no good.

Also, for the shape of the grooves in the diffraction optical surface ineach of the embodiments, there is the kinoform as shown in FIG. 11. Tomake a diffraction grating of the kinoform, a layer of ultravioletsetting resin is applied on the surface of a substrate 1 to form a resinpart 2, and in the resin part 2, there is formed a diffraction grating 3having such a grating thickness "d" that the diffraction efficiency inthe first-order diffracted light in a wavelength of 530 nm is 100%.

FIG. 12 shows the wavelength dependent characteristic of the diffractionefficiency in the first-order diffracted light in the diffractionoptical element shown in FIG. 11. As is apparent from FIG. 12, thediffraction efficiency in the design order gradually lowers as thewavelength deviates from the optimized one at 530 nm. In the meantime,the diffracted light of orders near to the design one increases. In FIG.12, the zero-order diffracted light and the second-order diffractedlight increase. This increase of the diffracted light of differentorders from the design one gives rise to a flare, which in turn lowersthe resolving power of the optical system.

Therefore, it is preferred to employ the laminated type of diffractionoptical element. As shown in FIG. 13, on the substrate 1 there areformed a first diffraction grating 4 made from an ultraviolet settingresin (Nd=1.499, υd=54) and, as is stacked thereon, a second diffractiongrating 5 made from another ultraviolet setting resin (Nd=1.598, υd=28)in mating relation.

For such a combination of the materials, the grating thickness d1 of thefirst diffraction grating 4 is determined to be 13.8 μm and the seconddiffraction grating 5 is stacked to a thickness d2 of 10.5 μm. Thediffraction optical element of the above structure has a wavelengthdependent characteristic of the diffraction efficiency in thefirst-order diffracted light, as shown in FIG. 14. As is apparent fromFIG. 14, owing to the laminated structure, the diffraction efficiencyfor the design order becomes higher values than 95% over the entireuseful range of wavelengths.

The use of the diffraction grating of such a laminated structureimproves the resolving power in the low frequencies, giving a greatadvantage of obtaining a desired optical performance. By using thediffraction grating of the laminated type for the diffraction opticalelement in each of the embodiments, therefore, the optical performanceis further improved.

It should be noted that the diffraction grating of the laminatedstructure described above is not confined in material to the ultravioletsetting resin. Other materials such as plastics may be used instead.Although depending on the kind of material to be used in the substrate,the first diffraction grating 4 itself may be made as the substrate.

Further, there is no need to make the grating thickness differ betweenthe first and second layers. For some combinations of materials, thegrating thicknesses in the two layers may be made equal to each other asthe thickness "d", as shown in FIG. 15. Since, in this case, no apparentgrating spaces are formed in the surface of the diffraction opticalelement, the dust proof is excellent, contributing to an increase of theproductivity on the assembling line in manufacturing the diffractiongratings. So, an inexpensive optical system can be obtained.

Next, eight numerical examples 1 to 8 are shown, which correspond to theembodiments 1 to 8 shown in FIGS. 3 to 10, respectively. In thenumerical data for the numerical examples 1 to 8, Ri is the radius ofcurvature of the i-th lens surface, when counted from the object side,Di is the i-th axial lens thickness or air separation, and Ni and υi arerespectively the refractive index and Abbe number of the material of thei-th lens element.

The aspheric coefficients K and A to E are given by the followingequation: ##EQU2## where X is the axial deviation from the vertex of thelens surface; H is the distance from the optical axis, and R is theradius of the osculating sphere.

Also, the values of the phase coefficients of up to eighth degree in theequation (1) are shown together with the values of the asphericcoefficients.

NUMERICAL EXAMPLE 1:

    ______________________________________                                        Numerical Example 1:                                                          ______________________________________                                        f = 4421.20  2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 59.162 D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.778 D2 = 6.80                                                          R3 = Fixed Stop D3 = 5.78                                                    *R4 = 10.746. D4 = 3.70 N2 = 1.491710 ν2 = 57.4                            *R5 = -4.780 D5 = 0.17                                                         R6 = ∞ D6 = 13.69 N3 = 1.524700 ν3 = 56.2                            R7 = -20.000 D7 = 1.50                                                        R8 = ∞ D8 = 24.00 N4 = 1.570900 ν4 = 33.8                            R9 = ∞ D9 = 0.81                                                       *R10 = 27.633 D10 = 2.35 N5 = 1.491710 ν5 = 57.4                            R11 = -13.366 D11 = 15.00                                                     R12 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = 1.59638 · 10.sup.2                                                            A = 0      B = 1.29316 · 10.sup.-3                                                C = -6.33288 · 10.sup.-5 D =                                       1.67969 · 10.sup.-6 E = 0                                             R5: K = -5.12549 · 10.sup.-1                                        A = 0 B = 9.46044 · 10.sup.-4                                          C = -9.69807 · 10.sup.-5 D =                                       1.02755 · 10.sup.-4 E = 0                                             R10: K = 1.02530 · 10.sup.-1                                        A = 0 B = -8.46894 · 10.sup.-5        C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase Coefficients:                                                               R4:      C2 = -4.22008 · 10.sup.-3                                                            C4 = -4.65602 · 10.sup.-5                  C6 = -4.42580 · 10.sup.-5 C8 = 1.83065 · 10.sup.-6       ______________________________________                                    

    ______________________________________                                        Numerical Example 2:                                                          ______________________________________                                        f = 4173.22  2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 104.689                                                                              D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 2.941 D2 = 4.70                                                          R3 = Fixed Stop D3 = 3.17                                                    *R4 = 9.886 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                              *R5 = -4.129 D5 = 0.17                                                         R6 = ∞ D6 = 13.69 N3 = 1.524700 ν3 = 56.2                            R7 = -20.000 D7 = 1.50                                                        R8 = ∞ D8 = 24.00 N4 = 1.570900 ν4 = 33.8                            R9 = ∞ D9 = 0.81                                                       *R10 = 27.633 D10 = 2.35 N5 = 1.491710 ν5 = 57.4                            R11 = -13.366 D11 = 15.00                                                     R12 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = -5.26499 · 10.sup.1                                                           A = 0      B = 1.51439 · 10.sup.-3                                                C = -1.59974 · 10.sup.-5 D =                                       2.16369 · 10.sup.-6 E = 0                                             R4: K = -2.67765 · 10.sup.1 A                                       = 0 B = 1.29504 · 10.sup.-3                                            C = 1.63472 · 10.sup.-4 D =                                        -4.72881 · 10.sup.-5 E = 0                                            R10: K = 1.02530 · 10.sup.-1                                        A = 0 B = -8.46894 · 10.sup.-5        C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase Coefficients:                                                               R5:      C2 = -5.21488 · 10.sup.-3                                                            C4 = 7.55564 · 10.sup.-4                   C6 = -6.14140 · 10.sup.-5 C8 = -8.33074 · 10.sup.-6      ______________________________________                                    

    ______________________________________                                        Numerical Example 3:                                                          ______________________________________                                        f = -48855.05                                                                              2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 8.931  D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                *R2 = 3.332 D2 = 4.00                                                          R3 = Fixed Stop D3 = 2.83                                                    *R4 = 10.994 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = -4.436 D5 = 0.29                                                         R6 = ∞ D6 = 13.69 N3 = 1.524700 ν3 = 56.2                            R7 = -20.000 D7 = 1.50                                                        R8 = ∞ D8 = 24.00 N4 = 1.570900 ν4 = 33.8                            R9 = ∞ D9 = 0.80                                                       *R10 = 27.633 D10 = 2.35 N5 = 1.491710 ν5 = 57.4                            R11 = -13.366 D14 = 15.00                                                     R12 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric coefficients:                                                          R1:     K = -2.16745 · 10.sup.0                                                           A = 0      B = 3.08491 · 10.sup.-3                                                C = -2.15513 · 10.sup.-4 D =                                       -2.92823 · 10.sup.-5 E = 0                                            R5: K = -1.03308 · 10.sup.-0                                        A = 0 B = 1.42100 · 10.sup.-3                                          C = -4.51809 · 10.sup.-5 D =                                       1.24654 · 10.sup.-6 E = 0                                             R10: K = 1.02530 · 10.sup.-1                                        A = 0 B = -8.46894 · 10.sup.-5        C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase Coefficients:                                                               R2:      C2 = -4.67996 · 10.sup.-3                                                            C4 = 3.09325 · 10.sup.-3                   C6 = -6.72250 · 10.sup.-4 C8 = -1.07600 · 10.sup.-4                                    R4: C2 = -2.14316 · 10.sup.-3 C4 =                                  -1.97163 · 10.sup.-5                       C6 = 8.88126 · 10.sup.-6 C8 = -2.47331 · 10.sup.-7       ______________________________________                                    

    ______________________________________                                        Numerical Example 4:                                                          ______________________________________                                        f = -62245.73                                                                              2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 9.809  D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.799 D2 = 3.64                                                          R3 = Fixed Stop D3 = 4.00                                                    *R4 = 12.510 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = -5.125 D5 = 0.57                                                         R6 = ∞ D6 = 1.00 N3 = 1.491710 ν3 = 57.4                            *R7 = ∞ D7 = 0.50                                                        R8 = ∞ D8 = 13.69 N4 = 1.524700 ν4 = 56.2                            R9 = -20.000 D9 = 1.50                                                        R10 = ∞ D10 = 24.00 N5 = 1.570900 ν5 = 33.8                          R11 = ∞ D11 = 0.80                                                     *R12 = 27.633 D12 = 2.35 N6 = 1.491710 ν6 = 57.4                            R13 = -13.366 D13 = 15.00                                                     R14 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric coefficients:                                                          R1:     K = -1.09315 · 10.sup.1                                                           A = 0      B = 2.41117 · 10.sup.-3                                                C = -2.38385 · 10.sup.-4 D =                                       1.57103 · 10.sup.-5 E = 0                                             R4: K = -9.90658 A = 0 B = 1.48879                                           · 10.sup.-4                           C = -4.22557 · 10.sup.-5 D = 2.08627 · 10.sup.-6 E =                                       0                                             R5: K = -6.87681 · 10.sup.-1 A = 0 B = 2.68812 ·                                            10.sup.-4                                      C = -2.11883 · 10.sup.-5 D = 1.10532 · 10.sup.-6 E =                                       0                                             R12: K = 1.02530 · 10.sup.-1 A = 0 B = -8.46894 ·                                           10.sup.-5                                      C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase Coefficients:                                                               R7:      C2 = -4.08837 · 10.sup.-3                                                            C4 = 2.71758 · 10.sup.-4                   C6 = -1.08630 · 10.sup.-5 C8 = 3.28603 · 10.sup.-8       ______________________________________                                    

    ______________________________________                                        Numerical Example 5:                                                          ______________________________________                                        f = -77892.37                                                                              2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 10.667 D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.555 D2 = 3.64                                                          R3 = Fixed Stop D3 = 4.00                                                    *R4 = 11.637 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = -5.042 D5 = 0.57                                                        *R6 = ∞ D6 = 1.00 N3 = 1.491710 ν3 = 57.4                             R7 = ∞ D7 = 0.50                                                        R8 = ∞ D8 = 13.69 N4 = 1.524700 ν4 = 56.2                            R9 = -20.000 D9 = 1.50                                                        R10 = ∞ D10 = 24.00 N5 = 1.570900 ν5 = 33.8                          R11 = ∞ D11 = 0.80                                                     *R12 = 27.633 D12 = 2.35 N6 = 1.491710 ν6 = 57.4                            R13 = -13.366 D13 = 15.00                                                     R14 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = -1.06911 · 10.sup.1                                                           A = 0      B = 2.02045 · 10.sup.-3                                                C = -1.74030 · 10.sup.-4 D =                                       1.18441 · 10.sup.-5 E = 0                                             R4: K = -1.60133 A = 0 B = -1.06944                                          · 10.sup.-5                           C = -3.14380 · 10.sup.-5 D = 3.17951 · 10.sup.-6 E =                                       0                                             R5: K = -5.88602 · 10.sup.-1 A = 0 B = 6.59444 ·                                            10.sup.-4                                      C = -2.60151 · 10.sup.-5 D = 2.86623 · 10.sup.-6 E =                                       0                                             R12: K = 1.02530 · 10.sup.-1 A = 0 B = -8.46894 ·                                           10.sup.-5                                      C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase coefficients:                                                               R6:      C2 = -3.75905 · 10.sup.-3                                                            C4 = 2.61229 · 10.sup.-4                   C6 = -1.23791 · 10.sup.-5 C8 = 3.95917 · 10.sup.-8       ______________________________________                                    

    ______________________________________                                        Numerical Example 6:                                                          ______________________________________                                        f = -48910.29                                                                              2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 11.130 D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.685 D2 = 3.79                                                          R3 = Fixed Stop D3 = 4.00                                                    *R4 = 11.209 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = -5.012 D5 = 0.16                                                        *R6 = ∞ D6 = 1.00 N3 = 1.491710 ν3 = 57.4                            *R7 = ∞ D7 = 0.50                                                        R8 = ∞ D8 = 13.69 N4 = 1.524700 ν4 = 56.2                            R9 = -20.000 D9 = 1.50                                                        R10 = ∞ D10 = 24.00 N5 = 1.570900 ν5 = 33.8                          R11 = ∞ D11 = 0.80                                                     *R12 = 27.633 D12 = 2.35 N6 = 1.491710 ν6 = 57.4                            R13 = -13.366 D13 = 15.00                                                     R14 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = -1.01637 · 10.sup.1                                                           A = 0      B = 1.89060 · 10.sup.-3                                                C = -1.58334 · 10.sup.-4 D =                                       1.15117 · 10.sup.-5 E = 0                                             R4: K = -3.54166 · 10.sup.-1                                        A = 0 B = 1.20211 · 10.sup.-4                                          C = -4.96249 · 10.sup.-5 D =                                       2.3396 · 10.sup.-6 E = 0                                              R5: K = -5.87434 · 10.sup.-1                                        A = 0 B = 9.28063 · 10.sup.-4                                          C = -2.70017 · 10.sup.-5 D =                                       2.21565 · 10.sup.-6 E = 0                                             R12: K = 1.02530 · 10.sup.-1                                        A = 0 B = -8.46894 · 10.sup.-5        C = -2.01808 · 10.sup.-7 D = 0 E = 0                              Phase Coefficients:                                                               R6:      C2 = -3.73706 · 10.sup.-3                                                            C4 = 1.87830 · 10.sup.-4                   C6 = -3.08445 · 10.sup.-5 C8 = -3.48661 · 10.sup.-7                                    R7: C2 = 1.62750 · 10.sup.-4 C4 =                                   1.14820 · 10.sup.-4                        C6 = 1.07176 · 10.sup.-5 C8 = 1.04286 · 10.sup.-6        ______________________________________                                    

    ______________________________________                                        Numerical Example 7:                                                          ______________________________________                                        f = 7891.74  2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 8.833  D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.406 D2 = 4.00                                                          R3 = ∞ D3 = 3.38                                                       *R4 = 12.122 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = -4.687 D5 = 1.74                                                         R6 = ∞ D6 = 13.69 N3 = 1.524700 ν3 = 56.2                           *R7 = -20.000 D7 = 1.50                                                        R8 = ∞ D6 = 24.00 N4 = 1.570900 ν4 = 33.8                            R9 = ∞ D9 = 0.80                                                       *R10 = 27.633 D10 = 2.35 N5 = 1.491710 ν5 = 57.4                            R11 = -13.366 D11 = 15.00                                                     R12 = Eye Point D12 = 0.00                                                 ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = -1.12926 · 10.sup.1                                                           A = 0      B = 2.07521 · 10.sup.-3                                                C = -3.43894 · 10.sup.-5 D =                                       6.32235 · 10.sup.-8 E = 0                                             R4: K = -1.92334 · 10.sup.1 A                                       = 0 B = 6.79073 · 10.sup.-4                                            C = -1.29333 · 10.sup.-5 D =                                       1.71868 · 10.sup.-6 E = 0                                             R5: K = -7.67453 · 10.sup.1 A                                       = 0 B = 1.05929 · 10.sup.-3                                            C = -2.35408 · 10.sup.-5 D =                                       1.98857 · 10.sup.-6 E = 0                                             R10: K = 1.02530 · 10.sup.1 A                                       = 0 B = -8.46894 · 10.sup.-5                                           C = -2.01808 · 10.sup.-7 D =                                       0 E = 0                                     Phase Coefficients:                                                               R7:      C2 = -8.93930 · 10.sup.-3                                                            C4 = -3.49234 · 10.sup.-5                  C6 = -5.18434 · 10.sup.-8 C8 = -9.97089 · 10.sup.-11     ______________________________________                                    

    ______________________________________                                        Numerical Example 8:                                                          ______________________________________                                        f = 11658.22 2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 8.634  D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.252 D2 = 2.00                                                          R3 = Fixed Stop D3 = 1.00                                                    *R4 = ∞ D4 = 1.00 N2 = 1.491710 ν2 = 57.4                             R5 = ∞ D5 = 1.56                                                       *R6 = 10.104 D6 = 3.70 N3 = 1.491710 ν3 = 57.4                             *R7 = -4.404 D7 = 0.20                                                         R8 = ∞ D8 = 13.69 N4 = 1.524700 ν4 = 56.2                            R9 = -20.000 D9 = 1.50                                                        R10 = ∞ D10 = 24.00 N5 = 1.570900 ν5 = 33.8                          R11 = ∞ D11 = 0.80                                                      R12 = 27.633 D12 = 2.35 N6 = 1.491710 ν6 = 57.4                            R13 = -13.366 D13 = 15.00                                                     R14 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:    K = -1.41148 A = 0      B = 1.26575 · 10.sup.-3                                                C = -1.86444 · 10.sup.-4 D =                                       -1.01106 · 10.sup.-5 E = 0                                            R6: K = -1.12848 · 10.sup.1 A                                       = 0 B = 1.52094 · 10.sup.-3                                            C = 2.71771 · 10.sup.-6 D =                                        1.85623 · 10.sup.-6 E = 0                                             R7: K = -9.89707 · 10.sup.-1 A                                      = 0 B = 1.30952 · 10.sup.-3                                            C = -6.12826 · 10.sup.-5 D =                                       7.60363 E = 0                                  R12: K = 1.02530 · 10.sup.-1 A = 0 B = -8.46894 ·                                          10.sup.31 5                                     C = -2.01808 · 10.sup.-7 D = 0 E = 0                              ______________________________________                                        Phase Coefficients:                                                               R4:      C2 = -6.81258 · 10.sup.-3                                                            C4 = 3.74344 · 10.sup.-5                   C6 = 5.31636 · 10.sup.-5 C8 = -1.57034 · 10.sup.-5       ______________________________________                                    

The values of the factors in the conditions (2) and (3) for thenumerical examples 1 to 8 are listed in the following table.

    ______________________________________                                        Numerical                                                                       Example Condition No.                                                       No.            (2)       (3)                                                  ______________________________________                                        1              -1.80 · 10.sup.-6                                                              0.00170                                                2  -100 · 10.sup.-6 0.00269                                          3 -5.80 · 10.sup.-6 0.00248                                          4 -3.30 · 10.sup.-6 0.00099                                          5 -2.60 · 10.sup.-6 0.00097                                          6 -2.70 · 10.sup.-6 0.00079                                          7 -7.20 · 10.sup.-6 0.00379                                          8 -7.10 · 10.sup.-6 0.00215                                        ______________________________________                                    

The aberrations of the numerical examples 1 to 8 of viewfinder opticalsystems are shown in FIGS. 16A to 16D through FIGS. 23A to 23D,respectively, where g, d, C and F are the spectral g-line, d-line,C-line and F-line, and Δ-M is the meridional focus and AS is thesagittal focus.

FIG. 24 shows a real-image finder which is similar in construction tothe embodiments 1 to 8, but different in not making use of thediffraction optical surface or surfaces. The following table is thenumerical data of this conventional example. Its aberrations are shownin FIGS. 25A to 25D.

    ______________________________________                                        Conventional Example 1:                                                       ______________________________________                                        f = 4421.20  2ω = 55.6                                                                          Exit Pupil Diameter φ = 8.0                       ______________________________________                                        *R1 = 59.162 D1 = 1.30  N1 = 1.491710                                                                             ν1 = 57.4                                 R2 = 3.778 D2 = 6.80                                                          R3 = Fixed Stop D3 = 5.78                                                    *R4 = 10.746 D4 = 3.70 N2 = 1.491710 ν2 = 57.4                             *R5 = 10.746 D5 = 0.17                                                         R6 = ∞ D6 = 13.69 N3 = 1.524700 ν3 = 56.2                            R7 = -20.000 D7 = 1.50                                                        R8 = ∞ D8 = 24.00 N4 = 1.570900 ν4 = 33.8                            R9 = ∞ D9 = 0.81                                                       *R10 = 27.633 D10 = 2.35 N5 = 1.491710 ν5 = 57.4                            R11 = --13.366 D11 = 15.00                                                    R12 = Eye Point                                                            ______________________________________                                         *Aspheric Surface                                                        

    Aspheric Coefficients:                                                          R1:     K = 1.59638.102                                                                            A = 0      B = 1.29316 · 10.sup.-3                                                C = -6.33288 · 10.sup.-5 D =                                       1.67969 · 10.sup.-6 E = 0                                             R4: K = -1.02447                             R5: K = -5.12549 · 10.sup.-1 A = 0 B = 9.46044 ·                                            10.sup.-4                                      C = -9.69807 · 10.sup.-5 D = 1.02755 · 10.sup.-5 E =                                       0                                             R10: K = 1.02530 · 10.sup.-1 A = 0 B = -8.46894 ·                                           10.sup.-5                                      C = -2.01808 · 10.sup.-7 D = 0 E = 0                              ______________________________________                                    

The aberration curves shown in FIGS. 25A to 25D, as are apparent oncomparison with those in FIGS. 16A to 16D through FIGS. 23A to 23D ofthe embodiments 1 to 8, are inferior to those of the systems having thediffraction optical surface in the optical element. With respect to thelateral chromatic aberration, extreme deterioration results in theconventional example.

Next, three embodiments 9 to 11 different from the above-describedembodiments 1 to 8 are described below.

FIG. 26 schematically shows the construction and arrangement of theconstituent parts of a real-image viewfinder. In the viewfinder, thereare disposed a first lens unit L1, a second lens unit L2, a third lensunit L3, a first image inverting optical member L4, a field stop S, asecond image inverting optical member L5 and an eye-piece lens unit L6in this order from the object side. In FIG. 26, P is assumed to be aneye point. A and A' indicate a bundle of on-axial rays. B and B'indicate a bundle of rays whose maximum field angle is determined by amember for limiting the passage of light such as the field stop S, or amarking for defining the field of view, positioned at or near theprimary image plane. A principal ray C halves the bundle of raysindicated by B and B' on the meridional section. The rays from theprimary image plane are made almost parallel by the eye-piece lens unitL6.

In this instance, according to the embodiments 9 to 11, letting thedistance of the maximum zone in the on-axial bundle of rays (A, A') fromthe optical axis be denoted by H, and the distance from the optical axisof the principal ray C which halves the off-axial bundle of rays havingan effective maximum field angle be denoted by HD, a diffraction opticalsurface is disposed in a lens unit which satisfies the followingcondition:

    |HD">|H|                        (4).

When the diffraction optical surface lies within this range, the anglesof incidence of the on-axial and off-axial bundles on the diffractionoptical surface can be varied in such a manner as to minimize the sizeof common working area. For any field angle, therefore, the lateralchromatic aberration resulting from the variation of the imagemagnification in each wavelength with the angle of incidence iscorrected without giving too much influence to the ones for the otherfield angles. It is thus made possible to advantageously correct thelateral chromatic aberration.

The image inverting optical members L4 and L5 L4 take the form of aPechan roof prism like that shown in FIG. 27, but may otherwise beconstructed with a Porro-prism or like other forms of prisms, or withmirrors. The real-image viewfinder may otherwise be formed also to thesecondary image type, as schematically shown in FIG. 28, comprising anobjective lens unit L7 for varying the image magnification, an imagere-forming lens unit L8 for varying the image magnification and aneye-piece lens unit L6.

FIGS. 29 to 31 in lens block diagrams show the embodiments 9 to 11,respectively, in the wide-angle end, a middle focal length position andthe telephoto end, with the image inverting optical members L4 and L5 inthe developed form. The objective optical system comprises, in orderfrom the object side, a negative first lens unit L1, a negative secondlens unit L2 and a positive third lens unit L3. During zooming from thewide-angle end to the telephoto end, while the first lens unit L1remains stationary on the optical axis, the second and third lens unitsL2 and L3 move axially in such relation that the air separationtherebetween narrows.

In the embodiment 9, the front surface of a rear lens in the first lensunit L1, the front surface of a front lens and the front surface of arear lens in the third lens unit L3, and the front surface of theeye-piece lens unit L6 are formed to aspheric shapes. The frontmostsurface of the first lens unit L1 is provided with a diffraction opticalsurface.

In the embodiment 10, four aspheric surfaces are used at similarlocations to those of the embodiment 9. A diffraction optical surface isprovided on the rear surface of the front lens in the first lens unitL1.

In the embodiment 11, aspheric surfaces are applied to the front surfaceof the rear lens in the first lens unit L1, the front surface of therear lens in the third lens unit L3, and the front surface of theeye-piece lens unit L6. A diffraction optical surface is provided on thefront surface of the rear lens in the first lens unit L1.

Under the condition of the inequality (4) described above, determinationof a location for the diffraction optical surface is made in order toinsure that the features of the diffraction optical element are utilizedin producing an optical system for the viewfinder which is correctedparticularly well for, among other aberrations that determine whetherthe image looked through the viewfinder by the naked eye is good or bad,chromatic aberrations, especially lateral one, without increasing thenumber of constituent lens elements.

It is also preferable that, letting the paraxial refractive power of alens L_(h) having a diffraction optical surface be denoted by ψh, andthe coefficient of the term of second degree in the equation (1) for thediffraction optical surface be denoted by C2h, the following conditionis satisfied:

    ψh·C2h<0                                      (5).

The inequality (5) is a condition for correcting chromatic aberrationswell. Since the diffraction optical element of rotational symmetry hasan Abbe number υd of negative sign as described before, the coefficientin the second degree of the diffraction optical surface of the lensL_(h) is made to take the reverse sign to that of the combinedrefractive power of the lens L_(h) and the diffraction optical surface,so that the chromatic aberrations are corrected advantageously. So, itis desirable to correct the chromatic aberrations the lens L_(h)produces, in such a manner as described above.

Incidentally, even in the embodiments 9 to 11, it is preferred tosatisfy the conditions (2) and (3).

The viewfinder which satisfies the condition (4) is not confined to thistype of real-image viewfinder. It is to be understood that the inventionis applicable to many other types of zoom or mono-focal lengthviewfinders with production of similar advantages.

Although, in the embodiments 9 to 11, the diffraction optical surface isused in the objective lens system, it may be used in the eye-piece lenssystem or the image inverting optical system. Even in this case, animproved optical performance can be expected. Another diffractionoptical surface may be likewise added to one of the other lens surfacesin the objective lens system. In this case, even more improved resultscan be attained.

Next, three numerical examples 9 to 11 are shown, which correspond tothe embodiments 9 to 11, respectively. In the numerical data for theexamples 9 to 11, Ri is the radius of curvature of the i-th lenssurface, when counted from the object side, Di is the i-th axial lensthickness or air separation, and Ni and υi are respectively therefractive index and Abbe number of the material of the i-th lenselement.

    ______________________________________                                        Numerical Example 9:                                                          ______________________________________                                                                    Exit Pupil Diameter                                 f - 152019.03˜-5604.63 2ω = 62°˜30°                                     φ 32  8.8˜4.8                           ______________________________________                                        R1 = ∞ D1 = 1.40  N1 = 1.491710                                                                             ν1 = 57.4                                R2 = ∞ D2 = 1.85                                                        R3 = ∞ D3 = 3.00 N2 = 1.491710 ν2 = 57.4                             R4 = 19.626 D4 = Variable                                                     R5 = -4.322 D5 = 1.30 N3 = 1.49171D ν3 = 57.4                              R6 = -21.358 D6 = Variable                                                    R7 = 11.599 D7 = 2.80 N4 = 1.491710 ν4 = 57.4                              R8 = -12.444 D8 = 1.53                                                        R9 = ∞ D9 = 4.20 N5 = 1.491710 ν5 = 57.4                             R10 = -7.964 D10 = Variable                                                   R11 = ∞ D11 = 14.50 N6 = 1.570900 ν6 = 33.8                          R12 = ∞ D12 = 1.50                                                      R13 = ∞ D13 = 24.00 N7 = 1.570900 ν7 = 33.8                          R14 = ∞ D14 = 0.20                                                      R15 = 27.633 D15 = 2.50 N8 = 1.491710 ν8 = 57.4                            R16 = -12.281 D16 = 15.00                                                     R17 = Eye Point                                                             ______________________________________                                    

    ______________________________________                                        Variable Focal Length                                                         Separation                                                                             152019.93     -9987.36 -5604.63                                      ______________________________________                                        D4       2.39          3.54     2.34                                            D6 7.52 2.69 0.89                                                             D10 1.35 5.03 8.04                                                          ______________________________________                                    

    ______________________________________                                        Aspheric Coefficients:                                                          R3:     K = 0        A = 0      B = 7.86283.10                                 C = -2.35476 · 10.sup.-5 D = 9.26607 · 10.sup.-7 E =                                       0                                             R7: K = 0 A = 0 B = -1.23089 · 10.sup.-4                              C = -6.88633 · 10.sup.-6 D = 4.95196 · 10.sup.-8 E =                                       0                                             R9: K = 0 A = 0 B = -8.15588 · 10.sup.-4                              C = -9.81455 · 10.sup.-7 D = 0 E = 0                                R15: K = 0 A = 0 B = -9.65364 · 10.sup.-5                             C = -9.11764 · 10.sup.-8 D = 0 E = 0                              ______________________________________                                        Phase Coefficients:                                                               R1:      C2 = 2.30070 · 10.sup.-11                                                            C4 = 4.71032 · 10.sup.-5                   C6 = 5.65911 · 10.sup.-7 C8 = -2.06028 · 10.sup.-8       ______________________________________                                    

    ______________________________________                                        Numerical Example 10:                                                                                     Exit Pupil Diameter                                 f = 152015.95˜-5604.63 2ω = 62°˜30°                                     φ = 8.8˜4.8                             ______________________________________                                        R1 = ∞ D1 = 1.40  N1 = 1.491710                                                                             ν1 = 57.4                                R2 = ∞ D2 = 1.85                                                        R3 = ∞ D3 = 3.00 N2 = 1.491710 ν2 = 57.4                             R4 = 19.626 D4 = Variable                                                     R5 = -4.322 D5 = 1.30 N3 = 1.491710 ν3 = 57.4                              R6 = -21.358 D6 = Variable                                                    R7 = 11.599 D7 = 2.80 N4 = 1.491710 ν4 = 57.4                              R8 = -12.444 D8 = 1.53                                                        R9 = ∞ D9 = 4.20 N5 = 1.491710 ν5 = 57.4                             R10 = -7.964 D10 = Variable                                                   R11 = ∞ D11 = 14.50 N6 = 1.570900 ν6 = 33.8                          R12 = ∞ D12 = 1.50                                                      R13 = ∞ D13 = 24.00 N7 = 1.570900 ν7 = 33.8                          R14 = ∞ D14 = 0.20                                                      R15 = 27.633 D15 = 2.50 N8 = 1.491710 ν8 = 57.4                            R16 = -12.281 D16 = 15.00                                                     R17 = Eye Point                                                             ______________________________________                                    

    ______________________________________                                        Variable Focal Length                                                         Separation                                                                             152015.95     -9987.37 -5604.63                                      ______________________________________                                        D4       2.39          3.54     2.33                                            D6 7.52 2.69 0.89                                                             D10 1.35 5.03 8.04                                                          ______________________________________                                    

    ______________________________________                                        Aspheric Coefficients:                                                          R3:     K = 0        A = 2.01093 · 10.sup.-7                                                         B = 5.69397 · 10.sup.-4                                                C = -3.55099 · 10.sup.-6 D =                                       4.67785 · 10.sup.-7 E = 0                                             R7: K = 0 A = 0 B = -1.23089 ·                                       10.sup.-4                                     C = -6.88633 · 10.sup.-6 D = 4.95196 · 10.sup.-8 E =                                       0                                             R9: K = 0 A = 0 B = -8.15588 · 10.sup.-4                              C = -9.81455 · 10.sup.-7 D = 0 E = 0                                R15: K = 0 A = 0 B = -9.65364 · 10.sup.-5                             C = -9.11764 · 10.sup.-8 D = 0 E = 0                              Phase Coefficients:                                                               R2:       C2 = 9.91000 · 10.sup.-8                                                            C4 = -2.61282 · 10.sup.-5                  C6 = 5.55894 · 10.sup.-6 C8 = -1.07282 · 10.sup.-7       ______________________________________                                    

    ______________________________________                                        Numerical Example 11:                                                                                      Exit Pupil Diameter                                f = 152019.38˜-5604.63 2ω = 62°˜30°                                      φ 32  8.8˜4.8                          ______________________________________                                        R1 = ∞ D1 = 1.40   N1 = 1.491710                                                                            ν1 = 57.4                                R2 = ∞ D2 = 1.85                                                        R3 = ∞ D3 = 3.00 N2 = 1.491710 ν2 = 57.4                             R4 = 19.626 D4 = Variable                                                     R5 = -4.322 D5 = 1.30 N3 = 1.491710 ν3 = 57.4                              R6 = -21.358 D6 = Variable                                                    R7 = 11.599 D7 = 2.80 N4 = 1.491710 ν4 = 57.4                              R8 = -12.444 D8 = 1.53                                                        R9 = ∞ D9 = 4.20 N5 = 1.491710 ν5 = 57.4                             R10 = -7.964 D10 = Variable                                                   R11 = ∞ D11 = 14.50 N6 = 1.570900 ν6 = 33.8                          R12 = ∞ D12 = 1.50                                                      R13 = ∞ D13 = 24.00 N7 = 1.570900 ν7 = 33.8                          R14 = ∞ D14 = 0.20                                                      R15 = 27.633 D15 = 2.50 N8 = 1.491710 ν8 = 57.4                            R16 = -12.281 D16 = 15.00                                                     R17 = Eye Point                                                             ______________________________________                                    

    ______________________________________                                        Variable Focal Length                                                         Separation                                                                             152019.28     -9987.35 -5604.63                                      ______________________________________                                        D4       2.39          3.54     2.33                                            D6 7.52 2.69 0.89                                                             D10 1.35 5.03 8.04                                                          ______________________________________                                    

    ______________________________________                                        Aspheric Coefficients:                                                          R3:     K = 0        A = 0      B = -1.62237 · 10.sup.-4                                               C = -7.05311 · 10.sup.-6 D =                                       5.88948 · 10.sup.-8 E = 0                                             R9: K = 0 A = 0 B = -7.37946 ·                                       10.sup.-4                                     C = -1.16001 · 10.sup.-5 D = 7.47669 · 10.sup.-7 E =                                       0                                             R15: K = 0 A = 0 B = -9.65364 · 10.sup.-5                             C = -9.11764 · 10.sup.-8 D = 0 E = 0                              Phase Coefficients:                                                               R3:      C2 = 6.38228 · 10.sup.-12                                                            C4 = 5.27204 · 10.sup.-5                   C6 = -1.51650 · 10.sup.-7 C8 = -1.05204 · 10.sup.-9      ______________________________________                                    

The values of the factors in the conditions (2) to (4) for the numericalexamples 9 to 11 are listed in the following table.

    ______________________________________                                                Numerical Example                                                     Condition 9            10        11                                           ______________________________________                                        (4) H in Wide-                                                                              1.65         1.47    1.65                                          Angle End                                                                    (4) HD in Wide- 5.32 4.84 3.69                                                 Angle End                                                                  (2)           1.94 · 10.sup.-14                                                                 -8.37 · 10.sup.-11                                                           5.39 · 10.sup.-15                 (3)           0.0015       0.0015  0.0015                                     ______________________________________                                    

The aberrations of the embodiments 9 to 11 of viewfinder optical systemsare shown in FIGS. 32A to 32D through FIGS. 40A to 40D, respectively,where g, d, C and F are the spectral g-line, d-line, C-line and F-line,respectively, and AM is the meridional focus and AS is the sagittalfocus. Incidentally, an object is assumed to lie at 3 meters from thefrontmost lens surface and its height is assumed to be at the largest ofthe field angles in the zooming range.

For comparison with the embodiments 9 to 11, a conventional example isshown under a similar condition to that of the embodiment 9, butdifferent in that only the diffraction optical surface is removed. Thefollowing table is the numerical data of this conventional example. Itsaberrations are shown in FIGS. 41A to 41D through FIGS. 43A to 43D. Ascompared with the aberration curves of the embodiments 9 to 11, it isapparent that the lateral chromatic aberration is left not improved.

    ______________________________________                                        Conventional Example 2.                                                                                    Exit Pupil Diameter                                f = 152019 · 38˜-5604.63 2ω = 62°˜30.d                                 egree. φ = 8.8˜4.8                     ______________________________________                                        R1 = ∞  D1 = 1.40  N1 = 1.491710                                                                            ν1 = 57.4                                R2 = ∞ D2 = 1.85                                                        R3 = ∞ D3 = 3.00 N2 = 1.491710 ν2 = 57.4                             R4 = 19.626 D4 = Variable                                                     R5 = -4.322 D5 = 1.30 N3 = 1.491710 ν3 = 57.4                              R6 = -21.358 D6 = Variable                                                    R7 = 11.599 D7 = 2.80 N4 = 1.491710 ν4 = 57.4                              R8 = -12.444 D8 = 1.53                                                        R9 = ∞ D9 = 4.20 N5 = 1.491710 ν5 = 57.4                             R10 = -7.964 D10 = Variable                                                   R11 = ∞ D11 = 14.50 N6 = 1.570900 ν6 = 33.8                          R12 = ∞ D12 = 1.50                                                      R13 = ∞ D13 = 24.00 N7 = 1.570900 ν7 = 33.8                          R14 = ∞ D14 = 0.20                                                      R15 = 27.633 D15 = 2.50 N8 = 1.491710 ν8 = 57.4                            R16 = -12.281 D16 = 15.00                                                     R17 = Eye Point                                                             ______________________________________                                    

    ______________________________________                                        Variable Focal Length                                                         Separation                                                                             152019.38     -9987.35 -5604.63                                      ______________________________________                                        D4       2.39          3.54     2.34                                            D6 7.52 2.69 0.89                                                             D10 1.35 5.03 8.04                                                          ______________________________________                                    

    ______________________________________                                        Aspheric Coefficients:                                                        ______________________________________                                        R3:   K = 0        A = 0        B = 6.83625 · 10.sup.-4                 C = -2.08926 · 10.sup.-5 D = 7.97515 · 10.sup.-7 E =                                     0                                               R7: K = 0 A = 0 B = -1.23089 · 10.sup.-4                              C = -6.88633 · 10.sup.-3 D = 4.95196 · 10.sup.-8 E =                                     0                                               R9: K = 0 A = 0 B = -8.15588 · 10.sup.-4                              C = -9.81455 · 10.sup.-7 D = 0 E = 0                                R15: K = 0 A = 0 B = -9.65364 · 10.sup.-5                             C = -9.11764 · 10.sup.-8 D = 0 E = 0                              ______________________________________                                    

Even in the embodiments 9 to 11, it is preferred that the diffractionoptical surface is disposed on a surface other than the frontmost orrearmost surface of the viewfinder optical system as far as possible.Further, in order to maintain the diffraction efficiency at a high valueover the entire useful range of wavelengths, it is also preferable toemploy the diffraction grating shown in FIG. 13 or 15 on the diffractionoptical surface.

What is claimed is:
 1. A viewfinder optical system comprising:anobjective optical system for forming a real image of an object; and aneye-piece optical system for converting rays of light from the realimage into substantially parallel rays of light, wherein said objectiveoptical system is provided with an optical element having a diffractionoptical surface.
 2. A viewfinder optical system according to claim 1,wherein said diffraction optical surface is rotationally symmetric withrespect to an optical axis, wherein, letting a distance from the opticalaxis be denoted by H, a reference wavelength be denoted by λ, and aphase coefficient with a term in the 2.i-th degree of H be denoted byC2·i, a phase φ(H) of said diffraction optical surface is expressed bythe following expression:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.2·i)

and wherein, letting a phase coefficient with a term in the seconddegree of H for the j-th diffraction optical surface be denoted by C2j,and a paraxial refractive power and Abbe number of the k-th opticalelement included in said viewfinder optical system be denoted by ψk andυk, respectively, the following condition is satisfied: ##EQU3## where nis the number of diffraction optical surfaces in said viewfinder opticalsystem, and m is the number of optical elements in said viewfinderoptical system.
 3. A viewfinder optical system according to claim 2,satisfying the following condition: ##EQU4##
 4. A viewfinder opticalsystem according to claim 1, further comprising: an image invertingoptical system for converting the real image into a noninverted erectingimage.
 5. A viewfinder optical system according to claim 1, wherein saidobjective optical system consists of, in order from an object side, anobjective lens of negative refractive power, an image re-forming lens ofpositive refractive power and an inverting prism for changing an opticalpath.
 6. A viewfinder optical system according to claim 1, wherein saidobjective optical system consists of, in order from an object side, afirst lens unit of negative refractive power, a second lens unit ofpositive refractive power and a third lens unit of positive refractivepower.
 7. A viewfinder optical system comprising:an objective opticalsystem for forming an image of an object; and an eye-piece opticalsystem for converting rays of light from the image into substantiallyparallel rays of light, wherein at least one of said objective opticalsystem and said eye-piece optical system has a diffraction opticalsurface of rotational symmetry with respect to an optical axis, wherein,letting a distance from the optical axis be denoted by H, a referencewavelength be denoted by λ, and a phase coefficient with a term in the2.i-th degree of H be denoted by C2·i, a phase φ(H) of said diffractionoptical surface is expressed by the following expression:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.6 + . . . +C2·i·H.sup.2·i)

and wherein, letting a phase coefficient with a term in the seconddegree of H for the j-th diffraction optical surface be denoted by C2j,and a paraxial refractive power and Abbe number of the k-th opticalelement included in said viewfinder optical system be denoted by ψk andυk, respectively, the following condition is satisfied: ##EQU5## where nis the number of diffraction optical surfaces in said viewfinder opticalsystem, and m is the number of optical elements in said viewfinderoptical system.
 8. A viewfinder optical system according to claim 7,satisfying the following condition: ##EQU6##
 9. A viewfinder opticalsystem comprising: an objective optical system for forming an image ofan object; andan eye-piece optical system for converting rays of lightfrom the image into substantially parallel rays of light, wherein adiffraction optical element is disposed on an optical element whichsatisfies the following condition:

    |HD|>|H|

where H is a distance from an optical axis of a maximum zone in a bundleof on-axial light rays passing through optical elements which constitutesaid viewfinder optical system, and HD is a distance from the opticalaxis of an off-axial principal ray which halves a bundle of off-axiallight rays having an effective maximum field angle.
 10. A viewfinderoptical system according to claim 9, wherein said diffraction opticalsurface is rotationally symmetric with respect to an optical axis,wherein, letting a distance from the optical axis be denoted by H, areference wavelength be denoted by λ, and a phase coefficient with aterm in the 2mi-th degree of H be denoted by C2·i, a phase φ(H) of saiddiffraction optical surface is expressed by the following expression:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.6 + . . . +C2·i·H.sup.2·i)

and wherein, letting a refractive power of the optical element havingsaid diffraction optical surface be denoted by ψh, and a phasecoefficient of a term of second degree of H for said diffraction opticalsurface be denoted by C2h, the following condition is satisfied:

    ψh·C2h<0.


11. A viewfinder optical system according to claim 10, wherein, lettinga phase coefficient with a term in the second degree of H for the j-thdiffraction optical surface be denoted by C2j, and a paraxial refractivepower and Abbe number of the k-th optical element included in saidviewfinder optical system be denoted by ψk and υk, respectively, thefollowing condition is satisfied: ##EQU7## where n is the number ofdiffraction optical surfaces in said viewfinder optical system, and m isthe number of optical elements in said viewfinder optical system.
 12. Aviewfinder optical system according to claim 11, satisfying thefollowing condition: ##EQU8##
 13. A viewfinder optical system,comprising: an objective optical system for forming an image of anobject; andan eye-piece optical system for converting rays of light fromthe image into substantially parallel rays of light, wherein at leastone of said objective optical system and said eye-piece optical systemhas a diffraction optical surface rotationally symmetrical with respectto an optical axis, wherein, letting a distance from the optical axis bedenoted by H, a reference wavelength be denoted by λ, and a phasecoefficient with a term in the 2·i-th degree of H be denoted by C2·i, aphase φ(H) of said diffraction optical surface is expressed by thefollowing expression:

    φ(H)=(2π/λ)·(C2·H.sup.2 +C4·H.sup.4 +C6·H.sup.6 + . . . +C2·i·H.sup.2·i)

and wherein, letting a phase coefficient with a term in the seconddegree of H for the j-th diffraction optical surface be denoted by C2j,and a paraxial refractive power and Abbe number of the k-th opticalelement included in said viewfinder optical system be denoted by ψk andυk, respectively, the following condition is satisfied: ##EQU9## where nis the number of diffraction optical surfaces in said viewfinder opticalsystem, and m is the number of optical elements in said viewfinderoptical system.